https://en.wikipedia.org/wiki/ECMAScript_for_XML - Firefox had it too, but people at large just didn't want it, so it got removed. It got disabled for web pages with the release of Firefox 17, 6 months prior to the first release of React.
People didn't want it because browsers didn't support it (except FF, as you noted). Some of us had our fingers crossed that other browsers would pick it up.
